Flow control method and apparatus for enhancing the performance of web browsers over bandwidth constrained links

ABSTRACT

Flow control is applied to increasing the performance of a browser while pre-fetching Web objects while operating over bandwidth constrained links to increase the level of concurrency, thus reducing contention for limited bandwidth resources with increased levels of concurrency. Using an agent or a gateway to speed up its Internet transactions over bandwidth constrained connections to source servers. Assisting a browser in the fetching of objects in such a way that an object is ready and available locally before the browser requires it, without suffering congestion on any bandwidth constrained link. Providing seemingly instantaneous availability of objects to a browser enabling it to complete processing the object to request the next object without much wait.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 13/207,226, filed Aug. 10, 2011 now U.S. Pat. No. 8,108,457,which is currently allowed, which is a continuation of Ser. No.12/940,376, filed Nov. 5, 2010, which is now U.S. Pat. No. 8,010,693,which is a continuation of U.S. patent application Ser. No. 12/749,305,filed Mar. 29, 2010, which is now U.S. Pat. No. 7,860,998, which is acontinuation of U.S. patent application Ser. No. 11/122,868, which isnow U.S. Pat. No. 7,694,008, filed May 4, 2005 and granted Apr. 6, 2010,each of which is incorporated in their entirety herein by this referencethereto.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to improvements in the performance of computernetworks. More particularly, the invention relates to increasingperformance of HTTP over long-latency links by pre-fetching objectsconcurrently via aggregated and flow-controlled channels.

2. Description of the Prior Art

Internet Web pages embed many types of URLs and objects. Standardbrowsers that display Internet Web pages can fetch some of these objectsin parallel from their Web server repository. Schemes that use abrowser's capability to fetch objects in parallel reduce the time takenby a browser to display pages that have many embedded objects. However,the degree of parallelism is limited due to backward compatibility,effectiveness of TCP flow control, and simplicity of browserimplementation when downloading certain types of objects, such asJavaScripts and StyleSheets. The currently well-known Web browsersalways download JavaScripts in sequence and StyleSheets in a separatephase.

The sequential fetching of objects degrades the end user experience withsuch URLs in high latency and low bandwidth networks, such as radioaccess networks. In short, servers are waiting for new requests toarrive but clients do not issue them until previous correspondingresponses are received. This causes the link to be under used.

A general purpose cache on a browser is used to improve the time takento revisit the same Web page. However, the general purpose cache doesnot help the first visit to a Web page over a wireless link. Also, asubsequent revisit may not be of benefit if the browser has clearedparts of its cache to accommodate objects from subsequent fetches fromother Web sites. Thus, the use of a general purpose cache does notprovide a consistent and predictable improvement in performance.

The HTTP 1.1 Request Pipelining standard (RFC 2616) allows the degree ofconcurrency to be increased if it is properly employed when both endssupport it. However, modern browsers do not use it in an optimal fashionfor many reasons, such as backward compatibility, simplicity ofimplementation, and effectiveness of flow control. Thus, the standarddoes not change the browser behavior, and therefore fails to optimizethe link use at all times.

It would be advantageous to provide a method and apparatus thatmitigates the negative impact of sequential access in low bandwidth andhigh delay networks without altering browser behaviors.

SUMMARY OF THE INVENTION

The invention mitigates the negative impact of sequential access in lowbandwidth and high delay networks without altering browser behaviors.This is achieved by increasing the degree of parallelism over thelong-latency link by imposing a different downloading strategy, i.e.pre-fetch, from an agent to a gateway server. The issue of low degreesof parallelism is localized between the browser and the agent wherethere is virtually no latency and bandwidth limitation for HTTPtransactions. The issue, therefore, is alleviated.

In an embodiment, flow control is applied to increasing the performanceof a browser while pre-fetching Web objects while operating overbandwidth constrained links to increase the level of concurrency, thusreducing contention for limited bandwidth resources with increasedlevels of concurrency.

In one embodiment, the invention comprises a piece of software (client)that communicates with corresponding server software which readies thenecessary object to be available on the platform hosting the browser.Low utilization over long-latency links occurs when browsers try todownload objects from source Web servers in sequence or in a degree ofconcurrency that is less-than-optimal. This contributes to a majorlatency experienced by the end user. This latency can be greatly reducedby using the pre-fetching mechanism of this invention. Thus, theinvention increases performance of HTTP over long-latency links bypre-fetching objects concurrently via aggregated and flow-controlledchannels. The agent and gateway together assist a Web browser infetching HTTP contents faster from Internet Web sites over long-latencydata links. The gateway and the agent coordinate the fetching ofselective embedded objects in such a way that an object is ready andavailable on a host platform before the resident browser requires it.The seemingly instantaneous availability of objects to a browser enablesit to complete processing the object to request the next object withoutmuch wait. Without this instantaneous availability of an embeddedobject, a browser waits for its request and the corresponding responseto traverse a long delay link. The invention is not limited to cachehistory on a browser. It actively performs selective fetch from thegateway server in a highly concurrent fashion, prior to the browserrequesting a particular object. It does not require any support from Webbrowsers and end source servers. It also does not introduce any flowcontrol issue under the high degree of parallelism. Thus, the inventionimproves HTTP performance of client-server implementations, especiallywith Web sites that use many JavaScripts and Style Sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block schematic diagram of a system architectureimplementing a method and apparatus for increasing performance of HTTPover long-latency links according to the invention;

FIG. 2 is a block schematic diagram that shows an overview of themodules interaction according to the invention;

FIG. 3 is a flow diagram that shows connection flow via the local cacheaccording to the invention;

FIG. 4 is a flow diagram that shows connection flow from the local cacheaccording to the invention; and

FIG. 5 is a block schematic diagram that shows a pre-fetch post HTMLprocessing connection according to the invention.

DETAILED DESCRIPTION OF THE INVENTION Definitions

HTTP: Hyper Text Transfer Protocol—a TCP/IP based protocol to transferWeb pages, documents, etc. over the Internet.

HTML: Hyper Text Markup Language.

API: Application Programming Interface.

VTP: A Transport Protocol.

XML: Extension Hyper Text Markup Language.

As of today, publicly available HTTP browsers open a limited number ofconcurrent TCP connections to Web servers due to the decreased flowcontrol effect as the number of TCP connections increase. This limitsthe degree of concurrency for the downloading of page objects. TheHTTP1.1 Pipelining standard supra. tries to solve this problem byallowing the sending out of multiple requests through the same TCPconnection before their responses are received. However, due to backwardcompatibility and the limitations of browsers and servers, the degree ofconcurrency is often low.

In addition, several types of objects, such as JavaScripts and CascadingStyle Sheets, are often downloaded sequentially by browsers due toimplementation simplicity. This further decrease the degree ofconcurrency, and therefore, the use of the long-latency link, due to thetime spent on waiting for responses.

The preferred embodiment of the invention comprises an agent and gatewaysystem that, by using an independent flow control mechanism (VTP), isnot concerned by the number of TCP connections virtually opened betweenthe browser host platform and the source Web (or proxy) server, and thatis therefore useful for downloading objects in parallel. By using acache on the client side, Web objects can be downloaded in parallel assoon as the agent is aware of them, independent from the implementationof end browsers and source Web servers.

FIG. 1 is a block schematic diagram of a system architectureimplementing a method and apparatus for increasing performance of HTTPover long-latency links according to the invention. There are four majorentities in the preferred system: a browser 11, typically within a hostplatform 10; a client 12, comprising an agent with pre-fetchfunctionality 17 and a Web object cache 13; a server 14, incommunication with said client via a long latency link having a flowcontrolled channel 19; and an original Web server 16 which is incommunication with said server 14. In the prior art, a browser requestis translated into signal messages and flow control under a transportprotocol (VTP) and routed to the server. The server translates therequest back to an original, regular HTTP request to the original Webserver, and the response travels back to the server. Then it isprocessed, either compressed using lossy or non-lossy techniques as areknown in the art, and transferred through the VTP again back to theclient. The client provides a compressed message back to the browser.The preferred embodiment uses a UDP-based, VTP flow control andtransport protocol, discussed below (see, also, U.S. Pat. No. 6,529,516and U.S. Pat. No. 6,115,384, which are incorporated herein in theirentirety by this reference thereto).

Where the degree of parallelism is low and the number of requests islow, the invention identifies the fact that many browsers branchJavaScripts and Style Sheets in a sequential manner, rather than inparallel. Thus, the invention explicitly pre-fetches this type ofobject, with dramatically improved overall performance.

For example, a Web page may have several JavaScripts and Style Sheetsand many GIFs. These are some of the components of a Web page, and theyare used for rendering the Web page. The Web page may also compriseadditional HTML, JPEGs, and FLASH, etc.

The inventors have identified two objects, i.e. JavaScripts and StyleSheets, as major factors that slow the system down, although theinvention is not limited to the pre-fetching of just these objects. Abrowser may already perform some parallel fetching, but JavaScripts andStyle Sheets are not typically fetched, nor is any effort made todiscriminate in the type of object that is being fetched. By selectivelypre-fetching these certain objects, such as these types of objects, thesystem's performance is dramatically improved. Thus, the preferredembodiment implements client-side selective pre-fetching which, in thepresently preferred embodiment, comprises JavaScript and Style Sheetpre-fetching.

The invention preferably comprises pre-fetcher 17 and a cache 18components, the latter of which is introduced in the client part of thesystem. To pre-fetch, the system also comprises a parser, a pre-fetchhandler, and a storage system to keep the pre-fetched objects (discussedbelow in greater detail).

In operation, the browser requests an object. The object is sent backthrough the server to the client. If the object is a description of aWebpage e.g. HTML the client parses it for the agent and determines thatthe Web page has certain components in it. In the preferred embodiment,the parser determines if there are any JavaScripts or Style Sheets, forexample, in the Web page. If there are, rather than waiting for thoseobjects to be requested by the user's browser, those objects areobtained from the Web server by the agent. Thus, when a Web page comesback to the user, the Web page goes back to the browser, andconcurrently the page does not pause while the JavaScript is fetched.

The Web page is a description object. It specifies component objects.Main page is always the first object that comes back, and it describeswhat is needed to render a page. Most of the time, the browser fetchesJavaScript and Style Sheet objects one at a time. Every time such anobject is fetched, the user must wait for a time delay. The inventorshave observed that the browser does not do the same thing for the othertype of objects, such as GIFs, JPEGs, or FLASH. The typical browserfetches other types of objects in parallel, at least to a certaindegree, and somehow only fetches certain types of objects one at a time.

The browser parses the page description, and while the page descriptionis operated upon by the browser, the parsing element is also parsing thepage description to identify JavaScripts and Style Sheets. Theseparticular objects are of a type that are normally sequentially fetched.The pre-fetch mechanism is used concurrently with the browser to go tothe original Web server to fetch these objects while the browser isstill loading the page. The system fetches these objects and brings themover to the cache, so that these objects are now resident at the cache.As a result, rather than have to go out across the system with itsconcomitant latency, to get these objects to complete the building ofthe Web page sequentially, it is only necessary access the clientlocally to complete loading the Web page. This speeds the system upconsiderably. Thus, the invention uses parallelism and selectivepre-fetching to give the impression of reduced round trip.

Functional Specification Problem Definition

In current client architectures, HTML pages are fetched when requestedby the browser. In certain situations, the browser requests pages, suchas JavaScripts, sequentially. Sequential requests do not maximize VTP asthere is no parallelism. To maximize VTP, pre-fetch on the client isrequired.

Assumptions

Implementation of the invention assumes that the browser performs HTMLrequests sequentially for certain files, such as JavaScripts.

Requirements

The invention implements a client pre-fetch feature with the followingrequirements:

-   -   Properties configurable via XML:    -   HTML page priority based on browser request    -   Non-persistence over different running session    -   Perform HTML file pre-fetch

Functionality

The following functional components are required: post HTML processing,client side pre-fetch control, and file cache.

The post HTML processing module is for parsing an HTML page to retrievea list of URLs to pre-fetch, regardless of their file extension. Thislist of URLs is fed into the client pre-fetch control module.

The client side pre-fetch control module performs HTTP requests foridentified objects.

The file cache handles caching pre-fetch file locally on persistencestorage.

Usability

The invention is preferably implemented with a provision for end usersto enable or disable file fetch. For this feature, the GUI presents anoption to either enable or disable this feature. On a disable, the localcached file, as well as internal pre-fetch record, is cleared.

Design Specification

This discussion provides details of the design for the post HTMLprocessing, client pre-fetch control, and file cache modules.

Overview

FIG. 2 is a block schematic diagram that shows an overview of themodules interaction. In FIG. 2, the browser 11 interacts through the OSsocket layer 20 with protocol 25,” The OS socket layer 20 interacts withan application layer 21 that, in turn, interacts with a compressor layer23 and a transport mechanism 22. An HTTP processing module 24 is the keyto operation of the invention and, in addition to the application layer21 and compressor layer 23, interacts with a post HTML processing module27 and the client pre-fetch control module 26, which itself interactswith the file cache control 18.

Module Design

The following discussion describes the module design for each subsystem,i.e. post HTML processing, client pre-fetch control, and file cache. Ineach of FIGS. 3-5, discussed below, operational flow is indicated by anumeral and an asterisk, e.g. “1*.”

FIG. 3 is a flow diagram that shows connection flow via the local cache.When the user accesses the browser to view a Web page 1*, the OS socketlayer/VLSP in turn communicates with the application layer 2*. The postHTML processing module is accessed 3* and returns an output to theapplication layer 4*. The application then accesses the transport module5*, which fetches a Web page, and the file cache module 6*, whichreturns a Web page to the OS socket layer/VLSP 7*.

FIG. 4 is a flow diagram that shows connection flow from the localcache. When the protocol module is active it accesses the OS socketlayer/VLSP 1* which, in turn accesses the application layer 2*. Theapplication layer interacts with the post HTML processing module 3*, 4*,and the application layer then uses the transport module 5*, after whichit provides cache contents to the OS socket layer/VLSP 6*.

FIG. 5 is a block schematic diagram that shows a pre-fetch post HTMLprocessing connection. Here, the server accesses the OS socket layer.VLSP 1* which, in turn, communicates with the application layer 2* and,thence, the HTTP processing module 3*. Flow proceeds to the post HTMLprocessing module 4*, which interacts with the client pre-fetch controlmodule 5*. The client pre-fetch control module interacts with thetransport module 6* which, in turn, communicates with the protocolmodule 7*. The protocol module returns requested results to the OSsocket layer/VLSP 8*.

Post HTML Processing Module Design

The post HTML processing module produces a list of URLs to pre-fetch.Each URL is fed into the client pre-fetch control module.

Client Prefetch Control Module Design

The client prefetch control module contains the decision logic todetermine whether and when to fetch a page. Pre-Fetch URL is stored inan index linked list structure. Once it determines that an URL should bepre-fetched, it establishes a bypass connection to the local cachemodule. The cache address and port number is determined by querying thelocal cache module. When the connection is established and it receivesdata from the local cache, the connection is closed immediately.Therefore, the local cache must be configured to continue with thedownload even when the connection is closed.

VTP (UDP) Transport Design

Overview of Operation

The following discussion describes the design of a UDP based VTPprotocol of the type that may be used in connection with the practice ofa preferred embodiment of the invention.

This protocol provides sequenced, reliable delivery of data as with TCP.It however employs a different rate control mechanism that is bettersuited for environments with high bandwidth variation and packet lossrate. In addition this protocol also supports multiplexing severaltransport flows over a single flow controlled channel between two hosts.One of the main goals of this protocol is to perform better in thoseareas that TCP does not: high bandwidth, high delay, and/or high packetdrops. Some of the drawbacks of TCP over wireless include its smallinitial send window, large maximum send window size, and very aggressivecongestion control mechanisms.

This protocol provides reliable delivery of data as provided by TCP. Ithowever employs a different rate control mechanism that is better suitedfor environments with high bandwidth variation and packet loss rate. Inaddition, this protocol also supports multiplexing multiple applicationdata flows over a single channel between two hosts. This allows agreatly increased number of application conversation between two hostswithout the side effect of reduced flow-control effectivenessexperienced when employing a high number of TCP connections betweenthem.

Connection Establishment

UDP is connectionless, datagram oriented protocol. In VTP, a logicalconnection must be established between the client and server for one ormore of the following reasons:

-   -   to exchange sequence numbers that guarantee sequenced delivery;    -   to authenticate the client connecting at the server side;    -   to get any start up parameters from each end.

VTP connections between two hosts are setup before to any applicationlevel flow is setup. Once the VTP connection is established, anindividual application conversation between two VTP-connected hostsrequires no three-way hand-shaking. A VTP end-point redirectingmechanism allows TCP flows being redirected into this VTP tunnel withoutexperience long setup delay.

VTP connection setup includes authentication and sequence numberexchange to guarantee delivery of control packets and data packets fromindividual application flows (TCP flows).

A new connection is opened if there was no prior connection with theserver. A connection open request is initiated from the client bysending a REQ_CONN (request connection) packet to the server. Aconnection request identifier is sent in every REQ_CONN packet to matchREQ_CONN and REQ_ACK packets. The client should start a timer after itsends this packet. If the REQ_CONN timer goes off, send another REQ_CONNpacket with a different connection request identifier. If no response isreceived from other side after sending some n (configurable parameter)number of REQ_CONN packets, the client gives up and reports “cannotestablish connection” to the caller. After the client sends the REQ_CONNpacket, it changes the status of that connection from closed(VTP_CLOSED) to “connection in progress” (VTP_REQ_CONN) state.

The server opens a socket and binds to some well known port forlistening to client connect requests. Whenever the server gets aREQ_CONN packet, it should allocate a new real connection node and replyto REQ_CONN packet by sending REQ_CONN+ACK packet. At this point, theserver moves that connection into “connection established” (VTP_EST)state. Once the client gets the REQ_CONN+ACK packet, it can move itsreal connection node into “connection established” state.

The client acknowledges the server's REQ_CONN+ACK packet with an ACKpacket. Note that the client can deduce its round trip time to theserver from REQ_CONN and REQ_CONN+ACK packets and the server can deduceround trip time between itself and this client from REQ_CONN+ACK and ACKpacket from the client. Each end should get an RTT estimate as often asnecessary to know the network.

If the real connection is opened successfully, data can be sent on avirtual connection by calling ta_send( ).

Data Flow and Explicit Rate Flow Control

One of the goals of any transport layer is to send data efficiently andto get the maximum throughput possible from the network. Once aconnection is established, any of the intermediate routers between thesender and receiver might go down or some more new connections might beestablished that use one or more of links in the path. The sender shouldnever send any data that leads to congestion in the network. The amountof data that the sender can send without getting any feedback from thereceiver is called the “send window” and it is calculated as:Send_Window=(const1*bandwidth*delay)+(const2*N)where:

-   -   bandwidth is the bottleneck link bandwidth (explained below)    -   delay is the round trip time between sender and receiver    -   const1 is an integer constant to correct bandwidth and delay        estimations    -   const2 is an integer constant to account for dropped ACK packets    -   N is the maximum bytes received before sending ACK.

The throughput of a connection is limited by the amount of data thenetwork can carry and the amount of buffer space the receiver has tostore this data before it passes it up to the application layer. In theinvention, receiver buffers are not regarded as a constraint. Becausethe network is the bottleneck now, the sender should never send any moredata than the network can accept, taking into account buffers inintermediate routers and in the sending host. For example, a sender andreceiver with 1 Mbps link are interconnected by a 128 kbps link. Thethroughput is now constrained by the 128 kbps (16 KBytes/s) link. If thesender has, e.g. 2 MB of data to send, it should not burst out all ofthese data quickly because they can get dropped at the intermediaterouter. To be exact, the sender should not send any more than ˜16 KB inone second. This makes it clear that if the sender has knowledge ofbandwidth of the bottleneck link in the path from itself to thereceiver, it can regulate its sending rate so that no packets aredropped in between.

To continue the example in above paragraph, say the send window is 30KB. Because all the data are there to send, a complete send window worthof bytes can be sent at a time. This generates a burst of packets at thenext router and, if there is no buffer space to hold these packets, theyjust end up getting dropped. To avoid this, because the sender knows thebandwidth it can send those bytes in a controlled way. This is the flowcontrol aspect of transport. As long as the bandwidth available to thisconnection remains the same, the chances of dropping a packet are less.The send window is necessary to avoid network congestion and flowcontrol is necessary to avoid bursts.

Once the sender sends its window's worth of bytes, it closes its windowand it should wait for feedback from the receiver. Ideally, if thesender has data to send it should never stop sending it until all thedata reaches other end i.e. the sender should never close its sendwindow. The sender should open its send window again when it getsfeedback from the receiver saying it got some or all of the data thesender has sent. Note that the sender should not free the data in itssend window until it is positively acknowledged by the receiver. To keepthe send window always open and have room for more data, the receivershould send its feedback as frequently as necessary.

It is clear that the knowledge of bandwidth and trip time for thetransport to work efficiently. The following sections describe how VTPgets the bottleneck bandwidth and trip time.

Round Trip Time

Round trip time (RTT) is the time taken for a packet to reach the otherend and to come back. This time includes queuing, transmission delays atthe sender, queuing and processing delays at intermediate routers, andprocessing delay at the final host. Trip time (TT) is the time a packettakes from the sender to the receiver. VTP uses the following equationto deduce RTT:RTT=(time from last packet sent to ACK received)−(delay atreceiver)−(sizeof(pkt)/bandwidth)

RTT is measured only when the sender releases one or more packets inresponse to a SACK, i.e. positively ACKed. It is not measured when SACKsays the receiver did not receive anything because the second term inthe equation needs a data packet.

The following example shows RTT computation:

receiver sender send 1 2 <----time t1 send 3 4 <----time t2 send 5 6<----time t3 t4---> 1 t5---> 3 t6---> 6 t7---> SACK says receiver got 1,3, 6. SACK arrives <-- time t8 RTT@sender = (t8 - t3) - (t7 - t6) -(sizeof(pkt 6)/bandwidth).

RTT is used in computing the send window and is also used in estimatingthe retransmission timeout (explained later). The client transport,which initiated the connection (real), can get RTT at connection setupwith REQ_CONN and REQ_CONN+ACK packets. The server can deduce its RTT tothe client if it gets the client's ACK packet. If the client's ACKpacket is dropped, the server has to derive RTT using above equationfrom the first SACK from the receiver. This requires the sender to timestamp each packet sent. The SACK packet should carry time differencefrom its last data packet to this SACK.

Send Window Close Timeout (WTO)

As mentioned earlier, the sender should stop after sending the last datapacket that it got from the application layer or after it sends a fullwindow's worth of bytes. It is up to the sender to guarantee that eachand every byte sent by the application layer is delivered to thereceiver in sequence and without any error. The receiver sends feedbackabout the packets it received and packets that are not. If the feedbackabout received packets is dropped, the sender should have some way toknow about the packets received at the receiver. To ensure that thereceiver is getting the packets it is sending, after sending the lastpacket it should start a timer that expires in some time. This timeoutis called last packet ACK timeout (LTO). If the sender does not get anyfeedback within LTO time, it sends a “requesting sack” packet to makethe receiver send a feedback (when to send feedback is explained inSACK). The LTO timer should be set such that it does not expire tooearly or too late. It should not expire too early for two reasons:Feedback may already be on the way to the sender; Too early timeoutscause too many retransmits. It should not expire too late for the reasonthat the pipe is not carrying any packets from this connection anymore.The LTO equation is given below:LTO=K5*(RTT+bytes_sent/bandwidth)where:

-   -   K5—integer constant to adjust delay    -   RTT—sender's RTT    -   bytes_sent—total number of bytes sent    -   bandwidth—transmit rate.

When the sender has sent a full send window's worth of bytes, it has tostop sending any new packets. The transport layer should make sure thatthis never happens when it has more data to send in virtual connectionqueues. The fact that the send window is closed and the sender is notsending any packets implies that the sender is not using the pipe to itsfullest. The SACKs, acknowledging the packets received enable the senderto release ACKed bytes from its send window, thereby providing room fornew bytes to be sent.

The sender starts “send window close timer” whenever it has to stopafter sending full send window. The send window close timer (WTO) isgiven as:WTO=2^M*Const*RTTwhere:

-   -   M—maximum number of retries    -   Const—an integer constant    -   RTT—sender RTT to client.

When WTO expires, the sender should send a new packet called SEND_SACKrequesting the receiver to send its SACK. The sender tries sending theseSEND_SACK packets up to some configured number of times (M). If it doesnot get any feedback after the last SEND_SACK packet, the sender shouldclose all the virtual connections on that real connection, discard anypackets in real connection out bound queue, and release the realconnection.

If the network does not drop any packet, there might never be a timeouton the WTO timer.

Bandwidth

Bandwidth of a path is the time it takes to transmit some number ofbytes from the sender to the receiver. This time includes transmissiontime of the device plus propagation time of the link(s) in between. Alink is the medium, wire or wireless, between two hosts. VTP measuresthe bandwidth of the path by sending two packets, one after another,with no time difference at the sender. Depending on the bandwidth of thelinks they traverse, they both arrive with some time difference at thereceiver. The arrival time is the time when the receiver gets thepacket(s) out of UDP socket buffers via recvfrom( ) system call. Notethat on high speed links, e.g. 4 Mbps, two or more packets can arrivewith zero time difference. The following shows how one can get thebandwidth by sending two packets of size 1 KB each on a 4 KB/s link.

c11 is packet 1 from connection 1, c12 is packet 2 from connection 1 andc21 is packet from connection 2.bandwidth=(c12 arrival time−c11 arrival time)/c12 packet size

Ignoring propagation delays, it takes 0.25 seconds for a 1 KB packet totraverse a 4 KB/s link.

CASE1: sender | c12 | c11 | receiver bw = (0.5 − 0.25)/1KB = 4 CASE2:sender | c21 | c12 | c11 | receiver bw = (0.5 − 0.25)/1KB = 4 CASE3:sender | c12 | c21 | c11 | receiver bw = (0.75 − 0.25)/1KB = 2 CASE4:sender | c12 | c22 | c21 | c11 | receiver bw = (1 − 0.25)/1KB = 1.33

In CASE1, connection 1 is the only connection using the link, so it getscomplete bandwidth, and that is what the transport sees. In CASES 2, 3,and 4, a new connection c2 is also using the same link. At this time,ideally, receiving transport for connection 1 should detect a bandwidthof 2 KB/s. But, as shown above, this can fluctuate anywhere from 1.33KB/s to 4 KB/s depending upon how the packets of connection 1 getsspaced. VTP arrives closely at 2 KB/s by averaging the bandwidthcontinuously. The receiving transport should intimate its currentreceive rate via SACK.

If the time difference between two packets in sequence is zero, VTPmeasures that as default maximum bandwidth.

Selective Acknowledgment (SACK)

SACK is the feedback from the receiver. A receiver should send feedbackabout the data it received as frequently as it can for several reasons:

-   -   to keep the send window open at sender side    -   to let the sender know about the packets lost and received    -   to let the sender know about receiver's current receive rate.

A SACK should be sent only when necessary.

Processing ACKnowledgement

The receiver sends an acknowledgement in the following cases:

-   -   1. When RTO expires    -   2. When it receives 16 KB or 32 packets in sequence    -   3. Any pending ACK can be piggybacked with data packet    -   4. First out of sequence packet    -   5. Substantial change of bandwidth.

The first three ACKs contain information about lost/received datapackets; the last two could be sent even if no new data packets arereceived by the receiver. Upon receiving an ACK that has lost/receivedinformation, the sender should immediately free up the VBufs that areACKed. If the incoming ACK positively acknowledges everything inout_queue, i.e. sent-but-unacked, and if sender had something to send,it should clear the ‘next transmit timer’ and send the new data becausethe pipe is clearly empty.

(Re)transmission Policy

The sender can send a new packet if it fits in the current send window.Otherwise, the sender should leave the packet in the virtualconnection's out bound queue and transmit them when send window isopened by the next SACK. Whenever the sender transmits a new packet, itshould start a fresh timer that should expire at LTO time. LTO timeincludes the transmit time plus trip time. When this timer expires, thesender sends a REQUEST_SACK packet and restarts the timer. If the senderdoes not get any reply from the receiver after sending N (configurable)number of REQUEST_SACK packets, the sender must close the connectionwith the receiver. The sender should never retransmit until it thinksthe packet it sent should be out of network based on current bandwidthand latency. If a retransmit request is received within this flighttime, the sender ignores the retransmit request.

The sender should never send its full send window worth of bytes atonce. This might lead to a burst of packets at the next or intermediaterouter and may cause packet drops if there are insufficient buffers. Toavoid this, the sender must always control the flow of packets by time,spacing them according to the bandwidth and delay of the path.

Connection Termination

The application layer may terminate a virtual connection at any time.The transport layer should send a connection close packet with the typeof close, i.e. abort or graceful close. transport layer should drop anypackets going from this virtual connection towards other end and send anew packet (just the header is sufficient) of type TP_ABORT if theapplication is aborting its connection. Otherwise, it should send allthe packets and then close the VC. In the latter case, it sets theTP_CLOSE bit in the last outgoing packet of VC to convey that VC at thisis closed. Once a virtual connection is closed, no data are sent orreceived on that connection. The receiving transport layer informs theapplication layer that the other side has requested connection closure.Note that the real connection is never closed. A real connection isclosed only when one end of the connection dies.

Checksum Computation

The transport should guarantee that data are delivered to the other endwithout any errors. To detect bit errors in transmission, a checksum iscomputed over the entire packet before delivering the datagram to thereceiver. The receiver should recompute the checksum for the packetarrived and compare it the checksum in the packet. If the checksumdiffers, consider that as a corrupted packet and discard it. Thechecksum field in the header must be taken as zero before computing thechecksum. To compute the checksum, divide all the data into 16-bitquantities and compute the one's complement of their one's complementsum. This is similar to checksum computation in the Internet.

Authentication

Authenticity of the client and/or server can be verified byauthentication module once the real connection is established at thetransport layer, but before any data are transmitted between each end.Note that a real connection may not be established depending uponauthentication parameters. For example, a real connection is denied attransport level if the server desires to authenticate a client but theclient has authentication disabled or client cannot supportauthentication.

Security

Security is provided in the transport to avoid any malicious hackersending a packet that looks like a packet that is sent by the system.One way one could disrupt the real connection is by sending a boguspacket that is in sequence with an authentic packet sent by theclient/server. The goal of the security in VTP is not to leave any holesfor such man in the middle attacks. To avoid this, each end sends apseudo-random sequence number (PRSN) that can be decoded only by thepeer entities involved.

A client can enable security at the transport level from UI. If thetransport is not configured to be secure, it uses serial sequencenumbers for data transmission and the ACK process.

Although the invention is described herein with reference to thepreferred embodiment, one skilled in the art will readily appreciatethat other applications may be substituted for those set forth hereinwithout departing from the spirit and scope of the present invention.For example, the pre-fetch mechanism could perform a selective pre-fetchof any selected objects, not just those that are sequentially loaded.The pre-fetch mechanism could apply various heuristics, rules, oradaptive behavior to determine which objects are to be pre-fetchedselectively. Further, Web pages could include metatags that areinterpreted by the pre-fetch mechanism and that determine which objectsare to be selectively pre-fetched.

Accordingly, the invention should only be limited by the Claims includedbelow.

1. A flow control apparatus, comprising: a facility operatively coupledwith a source server via a browser and a resource-constrainedcommunications link; a flow control mechanism configured for selectivelypre-fetching page objects from said source server and for downloadingpage objects in an optimally parallel fashion from said source server tosaid browser via said communications link; a cache operatively coupledwith said browser that caches said pre-fetched page objects and cachespage objects that are downloaded by said mechanism; wherein saidfacility is configured for enhancing said routine browser operation by:invoking said flow control mechanism to pre-fetch, in an optimallyparallel fashion, one or more page objects that are identified asfetched according to routine browser operation, and making saidpre-fetched page objects available to said browser by said facilitybefore they would be if they were fetched according to routine browseroperation.
 2. The apparatus of claim 1, wherein said flow controlmechanism is configured for communicating with a corresponding servermechanism, and configured for readying necessary page objects to beavailable to said browser.
 3. The apparatus of claim 1, wherein saidflow control mechanism further comprises: a module for measuringbandwidth of a path by sending at least two packets, one after another,across aid long latency link; and a module for using said bandwidth totransmit packets over said long latency link in a controlled way.
 4. Theapparatus of claim 1, wherein said flow control mechanism furthercomprises: a mechanism for activating said flow control mechanism inresponse to an increased degree of concurrency produced by pipelining bysaid browser or server.
 5. The apparatus of claim 3, wherein said modulefor measuring bandwidth and said module for using said bandwidth areconfigured for using round trip time measures for operation of said flowcontrol mechanism.
 6. The apparatus of claim 3, wherein said module formeasuring bandwidth of a path is configured for recognizing buffercapacity limitation on nodes between said browser and server; and saidmodule for using said bandwidth to transmit packets is configured forconsidering buffer capacity limitations of intermediate nodes.
 7. Theapparatus of claim 1, wherein said facility comprises any of an agent, agateway, and a reader.
 8. A flow control method for displaying pagesthat are comprised of a plurality of page objects, fetched over one ormore communications links, said method comprising the steps of:operatively coupling a facility with a source server via said browserand a resource-constrained communications link; operatively coupling afetching mechanism with said facility; selectively pre-fetching, usingsaid fetching mechanism, said page objects in a optimally parallelfashion from said source server to said browser via said communicationslink; providing a cache, operatively coupled with said browser, intowhich page objects are downloaded in parallel as soon as said facilityis aware of them; and providing an independent flow control mechanismfor controlling said parallel downloads optimally from said sourceserver to said browser.
 9. The method of claim 8, wherein said step ofselectively pre-fetching further comprises the steps of communicatingwith said source server, and readying necessary page objects to beavailable to said browser.
 10. The method of claim 8, furthercomprising: measuring bandwidth of a path by sending at least twopackets, one after another, across aid long latency link; and using saidbandwidth to transmit packets over said long latency link in acontrolled way.
 11. The method of claim 8, wherein said facilitycomprises any of an agent, a gateway, and a reader.
 12. The method ofclaim 8, wherein said flow control method further comprises: activatingsaid flow control mechanism in response to an increased degree ofconcurrency produced by pipelining by said browser or server.
 13. Themethod of claim 10, wherein said step of measuring bandwidth and saidstep of using said bandwidth apply round trip time measures foroperation of said flow control mechanism.
 14. The method of claim 10,wherein said step of measuring bandwidth of a path recognizes buffercapacity limitation on nodes between said browser and server; and saidstep of using said bandwidth to transmit packets considers buffercapacity limitations of intermediate nodes.